The present invention relates to a planographic printing plate for direct printing and a method for the preparation thereof.
Direct planographic printing or direct lithography is a printing technique in which paper or other material to be printed is printed by direct transfer of the printing ink from an inked printing plate to the surface thereof without the use of a rubber blanket as an intermediate carrier of the printing ink from the planographic printing plate to the surface of the paper used in conventional offset printing. Direct lithography is widely practiced with a presensitized plate by use of dampening water for the printing of newspapers and the like.
Ordinary planographic printing plates are constructed by providing hydrophilic non-image areas and oleophilic, i.e. ink-receptive, image areas on the surface of a substrate plate made of a metal or other rigid materials such as aluminum. Printing with such a printing plate is carried out by first moistening the plate with dampening water so as that the hydrophilic areas alone are coated with water to become ink-repellent and then inking with a printing ink which adheres only to the oleophilic areas and is transferred to the paper to form printed images.
The above described printing method is usually called wet-process direct lithography since the method involves the step of water supply to the surface of the printing plate. The presence of water on the printing plate is, however, largely responsible for the problems in the printing method that the paper under press is moistened with unavoidable distortion or the printing ink becomes emulsified resulting in decreased printing density or non-uniformity of printing. Moreover, the printing pressure for the transfer of the printing ink cannot be sufficiently high with a planographic printing plate so that the quality of the printed images is sometimes poor with insufficient transfer of the printing ink as a result of the low printing pressure below necessity. Therefore, the wet-process direct lithorgraphy has been considered not to be suitable for obtaining high-quality printed materials.
On the other hand, there is also known a method of dry-process direct lithography which is a planographic printing without the use of the dampening water. The printing plate used in this dry-process direct lithography is constructed by providing non-image areas formed of an ink-repellent material such as a silicone resin and oleophilic, i.e. ink-receptive, image areas on the surface of a substrate plate made of a metal or other rigid material such as aluminum. Printing plates of this type require no dampening water since the ink-repellency and the ink-receptivity are the properties inherent to the surface of the plate.
Accordingly, the undesirable phenomena of distortion of the paper under press and the emulsification of the printing ink unavoidable in the wet-process direct lithography never take place in the dry-process direct lithography so that high printing density with stability is obtained. This method is, however, not free from the problem of the insufficient printing pressure as in the wet-process direct lithography so that difficulties are also encountered in preventing lowering of the quality of the printed images caused by the deficient transfer of the printing ink. Therefore, printed materials obtained by the dry-process direct lithography are not evaluated as a high-quality printed material although the general quality of the printed images is somewhat improved over the wet-process direct lithography.
Generally speaking, direct lithography is superior to letterpress printing but much inferior to offset printing for halftone photographic printing whereas it is somewhat superior to offset printing but inferior to letterpress printing for the printing of characters or other line drawings. Among conventionally practiced printing methods, offset printing can give the highest degree of smoothness of the printed materials with respect to the quality of the printed images.
The reason for the above superiority of the offset printing is the use of a so-called blanket. As is well known, offset printing is performed by first transferring the printing ink on the printing plate to the surface of a blanket made of a rubbery elastomer, from where the printing ink is retransferred to the surface of the paper or other material to be printed. Accordingly, the printing ink on the printing plate is transferred to the blanket evenly with good adaptability by virtue of the rubbery elasticity of the blanket material and retransfer of the printing ink to the surface of the paper is also carried out very smoothly owing to the compensation for the fine irregularity on the paper surface by the rubbery elasticity of the blanket material.
The reason for the inferiority of wet- or dry-process direct lithography for halftone photographic printing in comparison with offset printing is the rigidity of the substrate plate such as a metal and the printing ink adhering thereon is directly contacted with paper or other material having a rather rough surface. For example, papers have microscopically a coarse and stiff surface by the presence of paper textures or fibrous structure so that, when contacted with the surface of an also rigid printing plate, no good adaptation is obtained between them resulting in uneven transfer of the printing ink and consequently degraded quality of the printed material. A remedy for this defect of poor adaptability is the increase of the local printing pressure by raising the printing plate at the image areas approaching to the surface condition of a plate for letterpress printing or application of an extremely large printing pressure. Application of an extremely large printing pressure is not practical because of the shortened durability of the planographic printing plate to an impractical extent in addition to the problem in the printing press per se which is usually not designed to give such an extraordinary large printing pressure or operated only with frequent troubles under such an extreme condition. At any rate, therefore, high quality cannot be expected in the printed materials obtained by the conventional techniques of direct lithography.